This relates to package design and more particularly to patterns of electrical interconnects that are used for high performance devices such as high speed differential signaling transceiver pairs and memory interfaces. As is known in the art, such interfaces are typically implemented using a ball grid array (BGA) or a pin grid array (PGA) A BGA is an area array of solder balls or solder bumps that is located on the surface of a package. A PGA is an area array of pins underneath a package surface. The BGA or PGA is used to connect the package to the next level of package. See, R. R. Tummala, Fundamentals of Microsystems Packaging, pp. 67, 68, 279-281, 680-682, 925 (McGraw-Hill, 2001), which is incorporated by reference herein in its entirety. For convenience, the term “contacts” or “interconnects” will be used hereafter to refer to solder balls, solder bumps and interconnect pins as well as similar connectors
In designing high speed differential signaling interfaces, it is important to achieve nominal differential impedance and good isolation between the high speed differential signaling interconnects. The practice has been to use as many ground contacts as possible to surround each pair of high speed differential signaling contacts. This, however, results in the use of large numbers of ground contacts, leading to consequences such as the need for trade-offs between the number of ground contacts and the number of I/O contacts, trade-offs in the number of ground contacts versus performance, and/or increases in the size of the interconnect package.
In designing high speed differential signaling interfaces, one approach has been to locate the differential signaling pairs in the vicinity of ground contacts. FIG. 1 depicts one such arrangement comprising a rectilinear array 100 of rows 102 and columns 104 of contact sites 110 on a substrate 120. As shown in FIG. 1, columns 104 are parallel to an edge 122 of substrate 220. In the first four columns, pairs of transmitter and receiver contacts 130, 140 alternate row-by-row with pairs of ground contacts 150. The pairing of two transmitter contacts 130 or two receiver contacts 140 is symbolized by a solid line between the pair. In the fifth and sixth columns, the contacts are general purpose input/output (GPIO) contacts 160. In this arrangement, the ground contacts 150 are permanently connected to ground, which is sometimes referred to as “hard-grounded.” The GPIO contacts may be switched so that they are connected to ground in some applications such as when an adjacent signaling pair is being used for differential signal transmission or reception but are otherwise available for use as general purpose input/output contacts. A switchable ground connection is sometimes referred to a “soft-grounded.” This arrangement has the advantage that two full columns of GPIO contacts are available for use as general purpose contacts. However, it has the disadvantage that the GPIO contacts do not provide adequate shielding for the high speed serial interface of the differential signal contacts 130, 140 with the result that cross-talk jitter is too high.
FIG. 2 depicts an alternative arrangement comprising a rectilinear array 200 of rows 202 and columns 204 of contact sites 210 on a substrate 220. Again, columns 204 are parallel to an edge 222 of substrate 220. In the first four columns, pairs of transmitter and receiver contacts 230, 240 alternate row-by-row with pairs of ground contacts 250. Again, the pairing of two transmitter contacts 230 or two receiver contacts 240 is symbolized by a solid line between the pair. In the fifth column, the contacts are all ground contacts 250 that are permanently connected to ground (i.e., are hard-grounded); and in the sixth column, the contacts are all GPIO contacts 260. In this arrangement, the ground contacts of the fifth column provide adequate shielding for the high speed serial interface of the transmitter and receiver contacts 230, 240 but at the price of making the contacts of the fifth column unavailable for use for general purpose input/output.